net: dsa: ocelot: add PTP dependency for NET_DSA_MSCC_OCELOT_EXT

A new user of MSCC_OCELOT_SWITCH_LIB was added, bringing back an old
link failure that was fixed with e5f31552 ("ethernet: fix
PTP_1588_CLOCK dependencies"):

x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_ptp_enable':
ocelot_ptp.c:(.text+0x8ee): undefined reference to `ptp_find_pin'
x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_get_ts_info':
ocelot_ptp.c:(.text+0xd5d): undefined reference to `ptp_clock_index'
x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_init_timestamp':
ocelot_ptp.c:(.text+0x15ca): undefined reference to `ptp_clock_register'
x86_64-linux-ld: drivers/net/ethernet/mscc/ocelot_ptp.o: in function `ocelot_deinit_timestamp':
ocelot_ptp.c:(.text+0x16b7): undefined reference to `ptp_clock_unregister'

Add the same PTP dependency here, as well as in the MSCC_OCELOT_SWITCH_LIB
symbol itself to make it more obvious what is going on when the next
driver selects it.

Fixes: 3d7316ac

 ("net: dsa: ocelot: add external ocelot switch control")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Colin Foster <colin.foster@in-advantage.com>
Link: https://lore.kernel.org/r/20230209124435.1317781-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>